1. Field of the Invention
The present invention relates to coating compositions highly suitable for the protection of articles directly exposed to the elements, such as buildings and other structures. This invention relates also to a method of forming hydrophilic films using these compositions, and to coated articles having hydrophilic films obtained with these compositions.
2. Prior Art
Because of their corrosion resistance and weathering resistance, silicone resins and fluorocarbon resins are commonly employed in exterior coatings used on buildings and structures which are directly exposed to the elements. Coats composed of these resins are resistant to degradation by acid rain and long-term exposure to sunlight. These coatings had generally been thought to be endowed also with excellent antifouling properties, but long-term weathering resistance tests showed that, due to the influence of atmospheric grime, dust and grit, iron particles, acid rain and the like, contaminants gradually deposit onto the surface of the coat, causing a deterioration in its appearance that is indicative of the coatis inadequate antifouling properties.
One known method for providing a coat with improved antifouling properties makes use of a self-cleaning effect achieved by adding to the paint or coating an alkyl silicate which is a silanol group precursor, and rendering the surface of the applied coat hydrophilic, so that contaminants are washed from the coat by rainwater. Contaminant adsorption can be prevented for a while in this way, but the self-cleaning effect is short-lived because the hydrophilizing substance is readily carried away on account of poor water resistance, and contaminants eventually become firmly attached to the coat surface. Prior-art techniques for preventing the deposition of contaminants by hydrophilizing the coat surface were thus inadequate in the durability and persistence of the desired effect. Another approach for preventing the hydrophilizing substance from being carried away involves the introduction of organic functional groups, but the degree of hydrophilization is unsatisfactory (see JP-A 40907/1997, JP-A 40908/1997, JP-A 40909/1997, JP-A 40911/1997).
International Patent Application WO 96/29375 describes a recently discovered method for imparting hydrophilicity to a coat that involves adding a microparticulate photooxidation catalyst, typically titanium oxide, to the coating composition. Irradiation of the photooxidation catalyst-containing coat with UV-containing light renders the coat hydrophilic to an extent where the water-drop contact angle of the surface becomes 10 degrees or less. This is believed to be a hydrophilizing system in which electrons and holes form at the surface of the fine photocatalyst particles that have been exposed to light. Oxygen and water in the air are oxidized and reduced at the surface of these particles, generating active radical species which in turn partially decompose the silicone resin serving as the binder in the coat, forming hydrophilic silanol groups at the surface of the coat, where moisture in the air is then adsorbed. Deposited organic contaminants are self-cleaned from the coat by rainwater. The hydrophilic properties of the coat can be maintained because even the contaminating ingredients that adhere to the surface of the coat are completely degraded oxidatively by the radical species which are generated. In addition, the presence of numerous hydroxyl groups at the surface of the fine particles of the photooxidation catalyst, such as titanium oxide, allows the particles to bond securely to the binder resin and not be carried away from the coat, thus making it possible to semi-permanently sustain the antifouling properties.
One known example of the generation of radicals by a photooxidation catalyst is the degradative deterioration (or chalking) by sunlight of a paint film in which titanium oxide is used as a white pigment. The surface of pigment-grade titanium oxide is generally coated with a metal oxide such as silica or alumina, which prevents the formation of radicals to a considerable yet incomplete degree. In long-term exposure tests on paint films, the erosion of resin from about the pigment is due to degradation of the resin by the small amount of radicals that are generated. Therefore, the binder resin in a hydrophilizable, photooxidation catalyst-containing coat must not be readily subject to chalking, and only the surface-most layer of the coat must be attacked by radicals to induce a very slight degree of resin degradation for hydrophilization. Silicone resins and modified silicone resins are suitable as this type of resin.
The silicone resins used as binders in paints and coatings are generally diluted with organic solvents. Even the photooxidation catalyst-containing coating compositions in above-mentioned International Patent Application WO 96/29375 invariably contain an organic solvent for dissolving the silicone resin. However, problems associated with the use of organic solvents, such as environmental contamination, toxicity to workers during application, and fire and explosion hazards, have led to a heightened demand for organic solvent-free water-based coatings.
In the design of a water-based silicone resin coating, methods which use a water-insoluble silicone resin as an emulsion obtained by dispersion of the resin in water and methods which use a water-soluble silicone resin may be contemplated. Silicone resin emulsion coatings do not require hydrophilic groups within the resin, in addition to which they have excellent water resistance, making them suitable as water-based coatings. On the other hand, depending on the type of emulsifying agent used, they may be subject to a strong pH influence, resulting in poor stability. Moreover, the processes involved in the preparation of silicone resin emulsion coatings tend to be complex and to require the use of large-scale emulsifying equipment, which can lead to considerable expense. As for the commonly used water-soluble silicone resins, condensation reactions readily occur to induce gelation in pH regions other than the weakly acidic pH levels at which the silanol groups are stable, and especially at an alkalinity of pH 9 or more.
In addition, microparticulate photooxidation catalyst-containing suspensions generally remain stably dispersed at a strong acidity of pH 4 or less and at a strong alkalinity of pH 9 or more. However, on account of problems associated with rusting and corrosion in actual use, it is advantageous to make appropriate use of microparticulate photooxidation catalyst-containing suspensions having an alkaline pH.